Progression is usually considered as a third stage in the natural history of neoplasia, after promotion, but the distinction between these two stages is not always possible because the two processes are very frequently linked together. The boundaries between promotion and progression are ill defined because irreversible changes, required by Foulds in his definition of progression, do indeed occur during promotion. They involve modifications of gene expression, DNA strand breakage, or the formation of chromosomal abnormalities. Cases in point are those of circulating myelodysplastic cells as well as dysplasias of the uterine cervix in solid tumors. There has been considerable interest in the promotion and progression of tumors in the last few years, but no regional or national conference has dealth with the boundaries between promotion and progression to malignancy. This conference will not attempt to define the two areas that are being bound; rather, it will concentrate on the mechanistic similarities between promotion and progression. By assembling together investigators with various backgrounds, this conference will distinguish itself by focusing on the molecular biology, developmental oncology and genetic pathology aspects of "The Boundaries". The first session will be devoted to a discussion of "The Boundaries" in animal tissues, the most innovative material. The morphologic and molecular characteristics will be reviewed in multistage carcinogenesis in the liver, skin and breast. The role of development in "The Boundaries" will be debated in the next session and a Colloquium will attempt to develop concise statements about future research with the use of transgenic mice in studies of the boundaries. Another session will deal with alterations due to oncogenes and suppressor genes in "The Boundaries". The problem of genetic instability and its role in "The Boundaries" will be next, followed by a "Colloquium" on thecritical question of the point in time at which genetic instability occurs. Finally, the last session will treat the problem of the boundaries in humans, using the examples of melanoma, breast and lung. We believe that by emphasizing these compelling areas and with the participation of the recognized speakers we have brought together, we will be rewarded with a scientifically profitable conference.